golang-testing

What this skill does

**Persona:** You are a Go engineer who treats tests as executable specifications. You write tests to constrain behavior, not to hit coverage targets.

**Thinking mode:** Use `ultrathink` for test strategy design and failure analysis. Shallow reasoning misses edge cases and produces brittle tests that pass today but break tomorrow.

**Modes:**

- **Write mode** — generating new tests for existing or new code. Work sequentially through the code under test; use `gotests` to scaffold table-driven tests, then enrich with edge cases and error paths.
- **Review mode** — reviewing a PR's test changes. Focus on the diff: check coverage of new behaviour, assertion quality, table-driven structure, and absence of flakiness patterns. Sequential.
- **Audit mode** — auditing an existing test suite for gaps, flakiness, or bad patterns (order-dependent tests, missing `t.Parallel()`, implementation-detail coupling). Launch up to 3 parallel sub-agents split by concern: (1) unit test quality and coverage gaps, (2) integration test isolation and build tags, (3) goroutine leaks and race conditions.
- **Debug mode** — a test is failing or flaky. Work sequentially: reproduce reliably, isolate the failing assertion, trace the root cause in production code or test setup.

> **Community default.** A company skill that explicitly supersedes `samber/cc-skills-golang@golang-testing` skill takes precedence.

**Dependencies:**

- gotests: `go install github.com/cweill/gotests/gotests@latest`

# Go Testing Best Practices

This skill guides the creation of production-ready tests for Go applications. Follow these principles to write maintainable, fast, and reliable tests.

## Best Practices Summary

1. Table-driven tests MUST use named subtests -- every test case needs a `name` field passed to `t.Run`
2. Integration tests MUST use build tags (`//go:build integration`) to separate from unit tests
3. Tests MUST NOT depend on execution order -- each test MUST be independently runnable
4. Independent tests SHOULD use `t.Parallel()` when possible
5. NEVER test implementation details -- test observable behavior and public API contracts
6. Packages with goroutines SHOULD use `goleak.VerifyTestMain` in `TestMain` to detect goroutine leaks
7. Use testify as helpers, not a replacement for standard library
8. Mock interfaces, not concrete types
9. Keep unit tests fast (< 1ms), use build tags for integration tests
10. Run tests with race detection in CI
11. Include examples as executable documentation

## Test Structure and Organization

### File Conventions

```go
// package_test.go - tests in same package (white-box, access unexported)
package mypackage

// mypackage_test.go - tests in test package (black-box, public API only)
package mypackage_test
```

### Naming Conventions

```go
func TestAdd(t *testing.T) { ... }               // function test
func TestMyStruct_MyMethod(t *testing.T) { ... } // method test
func BenchmarkAdd(b *testing.B) { ... }          // benchmark
func ExampleAdd() { ... }                        // example
func FuzzAdd(f *testing.F) { ... }               // fuzz test
```

## Table-Driven Tests

Table-driven tests are the idiomatic Go way to test multiple scenarios. Always name each test case.

```go
func TestCalculatePrice(t *testing.T) {
    tests := []struct {
        name     string
        quantity int
        unitPrice float64
        expected  float64
    }{
        {
            name:      "single item",
            quantity:  1,
            unitPrice: 10.0,
            expected:  10.0,
        },
        {
            name:      "bulk discount - 100 items",
            quantity:  100,
            unitPrice: 10.0,
            expected:  900.0, // 10% discount
        },
        {
            name:      "zero quantity",
            quantity:  0,
            unitPrice: 10.0,
            expected:  0.0,
        },
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            got := CalculatePrice(tt.quantity, tt.unitPrice)
            if got != tt.expected {
                t.Errorf("CalculatePrice(%d, %.2f) = %.2f, want %.2f",
                    tt.quantity, tt.unitPrice, got, tt.expected)
            }
        })
    }
}
```

## Unit Tests

Unit tests should be fast (< 1ms), isolated (no external dependencies), and deterministic.

## Testing HTTP Handlers

Use `httptest` for handler tests with table-driven patterns. See [HTTP Testing](./references/http-testing.md) for examples with request/response bodies, query parameters, headers, and status code assertions.

## Goroutine Leak Detection with goleak

Use `go.uber.org/goleak` to detect leaking goroutines, especially for concurrent code:

```go
import (
    "testing"
    "go.uber.org/goleak"
)

func TestMain(m *testing.M) {
    goleak.VerifyTestMain(m)
}
```

To exclude specific goroutine stacks (for known leaks or library goroutines):

```go
func TestMain(m *testing.M) {
    goleak.VerifyTestMain(m,
        goleak.IgnoreCurrent(),
    )
}
```

Or per-test:

```go
func TestWorkerPool(t *testing.T) {
    defer goleak.VerifyNone(t)
    // ... test code ...
}
```

## testing/synctest for Deterministic Goroutine Testing

`testing/synctest` (Go 1.25+) provides deterministic tests for goroutines, timers, deadlines, and context cancellation. Time advances only when all goroutines are blocked, making ordering predictable.

When to use `synctest` instead of real time:

- Testing concurrent code with time-based operations (time.Sleep, time.After, time.Ticker)
- When race conditions need to be reproducible
- When tests are flaky due to timing issues

```go
import (
    "context"
    "testing"
    "testing/synctest"
    "time"
)

func TestContextTimeout(t *testing.T) {
    synctest.Test(t, func(t *testing.T) {
        const timeout = 5 * time.Second

        ctx, cancel := context.WithTimeout(t.Context(), timeout)
        defer cancel()

        time.Sleep(timeout - time.Nanosecond)
        synctest.Wait()
        if err := ctx.Err(); err != nil {
            t.Fatalf("before timeout: %v", err)
        }

        time.Sleep(time.Nanosecond)
        synctest.Wait()
        if err := ctx.Err(); err != context.DeadlineExceeded {
            t.Fatalf("after timeout: got %v, want DeadlineExceeded", err)
        }
    })
}
```

Use `synctest.Test` in Go 1.25+ and Go 1.26+. Do not use the old Go 1.24 experimental `synctest.Run` API in Go 1.25+ or Go 1.26+ code. If a module explicitly targets Go 1.24 and opts into `GOEXPERIMENT=synctest`, use the old API only as a compatibility fallback.

Key differences in `synctest`:

- `time.Sleep` advances synthetic time instantly when the goroutine blocks
- `time.After` fires when synthetic time reaches the duration
- All goroutines run to blocking points before time advances
- Test execution is deterministic and repeatable

## Test Timeouts

For tests that may hang, use a timeout helper that panics with caller location. See [Helpers](./references/helpers.md).

## Benchmarks

→ See `samber/cc-skills-golang@golang-benchmark` skill for advanced benchmarking: `b.Loop()` (Go 1.24+), `benchstat`, profiling from benchmarks, and CI regression detection.

Write benchmarks to measure performance and detect regressions:

```go
func BenchmarkStringConcatenation(b *testing.B) {
    b.Run("plus-operator", func(b *testing.B) {
        for b.Loop() {
            result := "a" + "b" + "c"
            _ = result
        }
    })

    b.Run("strings.Builder", func(b *testing.B) {
        for b.Loop() {
            var builder strings.Builder
            builder.WriteString("a")
            builder.WriteString("b")
            builder.WriteString("c")
            _ = builder.String()
        }
    })
}
```

Benchmarks with different input sizes:

```go
func BenchmarkFibonacci(b *testing.B) {
    sizes := []int{10, 20, 30}
    for _, size := range sizes {
        b.Run(fmt.Sprintf("n=%d", size), func(b *testing.B) {
            b.ReportAllocs()
            for b.Loop() {